Any of the drills using conventional tips has a relatively large outer diameter in order to maintain the balance of cutting resistance between a central edge and an outer peripheral edge. Described in U.S. Pat. No. 4,586,858 and Published Unexamined Japanese Patent Application No. 62-84912, by the inventor hereof, are drills which use tips of special shapes such that they can drill holes of relatively small diameters, and thus drills which use tips intended for drilling relatively small holes have been known publicly.
In a drill disclosed in U.S. Pat. No. 4,586,858, however, a plurality of tips are arranged constituting cutting edges, and portions without the tips are provided so that the cutting edges are partially discontinuous, whereby left- and right-hand cutting edges are well balanced in cutting resistance. In this case, however, the diameter of each work hole can be reduced by only a limited amount. In a drill described as an alternative embodiment, on the other hand, tips constituting cutting edges are specially worked so that a portion incapable of cutting work is arranged at part of the cutting edge, whereby the cutting resistance is well balanced. In this case, special processing of the cutting edges entails high cost.
Further, in a drill described in Published Unexamined Japanese Patent Application No. 62-84912, tips constituting cutting edges are also specially worked so that a portion incapable of cutting work is arranged at part of the cutting edge, whereby the cutting resistance is well balanced. This drill also entails high cost.